1. Field of the Invention
This invention generally relates to a hollow plastic product and its manufacturing method, and, in particular, to a hollow plastic product having a connector pipe, which may be used as a duct, hose, tube or the like for flowing a gas, such as air, or liquid, such as water and its manufacturing method. In particular, a hollow plastic product of the present invention may be advantageously used as a duct, hose, tube or the like of an air intake system or a fuel supply system to be used in combination with an internal combustion engine of an automobile.
2. Description of the Prior Art
Among the components of an air intake system of an automobile, blow molded products are used for ducts, hoses, tubes for introducing air into an internal combustion engine, a blowby hose for returning a blowby gas containing the oil components that are produced at the engine to these ducts, etc., and components for connecting the bypass pipe to the air intake duct during idling. Such hollow plastic products as these ducts, pipes and tubes are often added with a connector pipe for establishing a connection among these components for operating the engine and its associated control equipment. For example, it is so structured that the blowby gas containing the oil components produced at the engine is returned from the engine crank case to the air intake duct and the bypass pipe during idling is connected to the air intake duct, in which case a connector pipe is provided at each connection.
Heretofore, in the case of adding a connector pipe having such a function to a blow-molded hollow product, a connector pipe molded by injection molding was used as an insert in molding, or a connector pipe was thermally welded afterwards. In particular, in the case where the adherence provided by insert molding is poor and a required air-tightness cannot be attained, use was made of a method of welding a connector pipe to a blow-molded product.
These ducts, hoses and tubes generally differ in the outer diameter of a main body as well as the diameter of a connection portion depending on the size of a companion component to be connected and a required flow rate, and a connector pipe added to the main body is also required to have a diameter in compliance with its companion component to be connected. For this reason, there is a case in which a connector pipe to be added to such ducts, hoses and tubes which have been manufactured by blow molding is required to have a diameter larger than the outer diameter of the main body of such blow-molded ducts, hoses and tubes.
It is, of course, possible to mold such ducts, hoses and tubes with a portion, to which a connector pipe having a larger diameter to be added, partially increased in outer diameter when blow molding. However, when such a portion of partially increased in outer diameter is formed by blow molding, the blow-up rate of a parison, i.e., a cylinder of a molten resin, supplied at the time of molding increases so that the thickness of that portion of the main body becomes smaller than the rest. In this manner, since such a blow-molded portion which has been increased in outer diameter becomes thinner, if, for example, a connector pipe is to be added to that portion by insert molding, the adherence strength of the insert becomes poor so that there is an increased chance for the insert to be disassembled, or, in some cases, the parison, when blown up, breaks to thereby form a hole. In addition, in the case of adding a connector pipe by thermal welding, a thin portion of a blow-molded product may become so deformed due to the heat at a melting step of thermal welding that it is impossible to complete the thermal welding.
Besides, there is a case in which flexibility is required for blow-molded ducts, hoses and tubes. In this case, since use is made of a soft resin in forming a main body to be blow molded, if a connector pipe is to be added to such a flexible duct, hose or a tube, it will easily fall off because of poor adherence strength if added by insert molding, and thermal welding cannot be used because the flexible blow-molded main body would become softer and deform undesirably during the pressure bonding stage of thermal welding.
In such conventional methods as described above, if a connector pipe was added to a blow-molded hollow product of a duct, pipe or tube, in particular, both comparable in diameter, the portion of the product to which the connector pipe was added became thinner, so that such problems as reductions of adherence strength, formation of holes and incapability of using thermal welding were brought about. The structure and size of a connector pipe to be added to a blow-molded hollow product was limited. Moreover, in the case of a blow-molded duct, hose or tube having flexibility, since the main body of such a product was made of a soft resin, there was a disadvantage of poor adherence strength of a connector pipe when added and there was also a problem of incapability of using thermal welding because the main body became too soft when heat was added during thermal welding. Thus, the material to be used for the main body was also limited.
FIG. 4 illustrates a structure resulting from a conventional thermal welding process as an example. As shown, a connector pipe 55 is added to a blow-molded hose 11 by thermal welding. In this case, however, if the diameter of connector pipe 55 is larger, a welding area at a welding portion 33 becomes smaller, so that the welding or adherence strength is reduced. Thus, the design of connector pipe 55 is limited and there is a case in which a desired product cannot be produced. In addition, since the welding portion 33 of blow-molded hose 11 is defined by a portion where the parison is enlarged during a blow-molding process, it becomes thinner, which tends to make it difficult to carry out thermal welding properly.
Furthermore, if the blow-molded hose 11 is made of a soft resin material, there may be a case where thermal welding cannot be applied since the hose 11 may become too soft when heat is applied, thereby causing the hose to be deformed.